Reflective liquid crystal display

ABSTRACT

A reflective liquid crystal display includes a linear polarizer for converting natural light into linearly polarized light; a retardation film for converting the linearly polarized light into circularly polarized light; a liquid crystal layer for varying the phase of the light differently depending on the presence or absence of an electric field; a cholesteric liquid crystal color filter for selectively reflecting light received from the liquid crystal layer; and a black background for absorbing light passing through the color filter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.09/536,636, filed Mar. 28, 2000, now U.S. Pat. No. 6,693,689; whichclaims priority to Korean Patent Application No. 1999-11108, filed onMar. 31, 1999, and the benefit of Korean Patent Application No.1999-48411, filed on Nov. 3, 1999, each of which are hereby incorporatedby reference for all purposes as if fully set forth herein.

BACKGROUND OF THE RELATED ART

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device,and more particularly to a reflective LCD device including a cholestericliquid crystal (CLC) color filter.

2. Description of Related Art

In general, LCD devices are divided into reflective LCD devices andtransmissive LCD devices. The transmissive LCD device uses an internallight source such as a back light, while the reflective LCD device usesambient light.

Particularly, since the reflective LCD device uses ambient light, thebrightness of the display depends on circumstances. In an office, thereflective LCD device is lower in brightness than the transmissive LCDdevice and, accordingly the color purity of an absorption-type colorfilter used in the LCD should be sacrificed to increase the brightness.

FIG. 1 is a cross-sectional view of a conventional reflective liquidcrystal display.

As shown in FIG. 1, the liquid crystal panel includes a linear polarizer26, a retardation film 24, a diffuser film 22, a first substrate 10, acolor filter 20, a common electrode 18, a liquid crystal layer 16, areflective electrode 14 and a second substrate, each are stacked in theabove-described order.

The reflective electrode 14 reflects light transmitted from outside thedisplay and also functions as a pixel electrode. The reflectiveelectrode 14 and the common electrode 18 apply a voltage to the liquidcrystal layer 16 and change the orientation of liquid crystal molecules.The diffuser film 22 reduces a surface reflection of light and increasesa viewing angle. The retardation film 24 such as a λ/4 plate convertslinearly polarized light into circularly polarized light. Further, thelinear polarizer changes the natural light into linearly polarizedlight.

The reflective LCD device described above functions and acts as follows.

When natural light is incident into the LCD device, the natural light isconverted into linearly polarized light by the linear polarizer 26, thenconverted into circularly polarized light by the retardation film 24.The circularly polarized light is converted into linearly polarizedlight while passing through the liquid crystal layer 16 and is reflectedon the reflective electrode 14. The reflected polarized light isconverted into circularly polarized light while passing through theliquid crystal layer again, then passes through the color filter toproduce colored light.

The circularly polarized light is diffused to increase the viewing anglewhile passing through the diffuser film 22, then is converted again intolinearly polarized light while passing through the retardation film 24.The linearly polarized light is displayed to the user after passingthrough the linear polarizer 26 in the form of images.

FIG. 2 shows the state of light while it passes through each of thecomponents described above when an electric field is not applied to theliquid crystal layer.

The natural light is first converted into linearly polarized lightthrough the linear polarizer 26. The linearly polarized light is changedinto circularly polarized light through the retardation film 24. Thecircularly polarized light is converted again into linearly polarizedlight through the liquid crystal 16, then reflected by the reflectiveelectrode 14. The reflected linearly polarized light is changed intocircularly polarized light through the liquid crystal layer 16. Thecircularly polarized light is finally converted into linearly polarizedlight through the retardation film 24.

FIG. 3 shows the state of light while it passes through each of thecomponents described above when an electric field is applied to theliquid crystal layer.

The natural light is first converted into linearly polarized lightthrough the linear polarizer 26. The linearly polarized light is changedinto circularly polarized light through the retardation film 24. Thecircularly polarized light is not changed when passing through theliquid crystal 16 as an electric field is applied to the liquid crystal16, then reflected by the reflective electrode 14. The reflectedcircularly polarized light is not varied even when passing through theliquid crystal 16. The circularly polarized light is finally convertedinto linearly polarized light through the retardation film 24, thenabsorbed by the linear polarizer 26.

FIG. 4 is a graph illustrating the reflectivity of light with respect tothe incident light of the LCD device described above. In FIG. 4, theX-axis indicates a wavelength λ, and the Y-axis indicates areflectivity. Note that a dominant wavelength region is referred to asregion A and other wavelengths are referred to as region B. As shown inthe graph, though light's reflective index is relatively high in theregion A, because light reflection is also carried out in the region B,the color purity of the LCD is reduced. It is required that the colorpurity is reduced in order to increase the transmissivity of the colorfilter, but just lowering the color purity to increase the brightnesshas a limitation.

Further, since the LCD having the configuration described above has amulti-layered structure in which each layer, i.e., each componentdiffers from one another in reflective index, the intensity of the lightis reduced while the light passes through each component. For example,the intensity of the light first is reduced while passing through thelinear polarizer 26, and then also prominently is reduced after passingthrough the color filter 20, because part of the light is absorbed orreflected while passing through the color filter 20.

Further, though the observer can clearly see the image displayed due toa good contrast ratio in the center of the screen, the contrast ratiobecomes lower as it gets far from the center of the screen, therebydeteriorating the display characteristic.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a reflective liquidcrystal display device having increased brightness without sacrificingcolor purity.

Another object of the present invention is to provide a reflectiveliquid crystal display device having high color purity and an improvedcontrast ratio.

In order to achieve the objects, in a first embodiment, a reflectiveliquid crystal display including a linear polarizer for convertingnatural light into linearly polarized light; a retardation film forconverting the linearly polarized light into a circularly polarizedlight; a liquid crystal layer for varying the phase of the lightdifferently depending on the presence of an electric field; acholesteric liquid crystal color filter for selectively reflecting thelight from the liquid crystal layer; and a black background forabsorbing the light passing through the color filter.

The present invention also provides, in the first embodiment, areflective liquid crystal display including first and second substratesopposite to and spaced apart from each other; a liquid crystal layerinterposed between the first and the second substrates, the liquidcrystal layer having a first switching mode in which the phase of lightis changed while passing through it and a second switching mode in whichthe phase of light is not changed while passing through it; first andsecond electrodes for applying an electric field to the liquid crystallayer; a semiconductor element located on the second substrate, forswitching an electric signal applied to the liquid crystal layer; aretardation film located on the first substrate, for converting alinearly polarized light a circularly polarized light; a linearpolarizer located on the retardation film, for converting natural lightinto the linearly polarized light; a cholesteric liquid crystal colorfilter located on the second substrate, for selectively reflecting thelight from the liquid crystal layer as a light of at least one color;and a black background located beneath the second substrate, forabsorbing the light passing through the color filter.

The retardation film is a λ/4 plate. The black background is locatedbeneath the color filter. The retardation film is located between thelinear polarizer and the color filter. The black background is made of apolymeric material. The color filter is designed so that a wavebandwidth of the color filter can be controlled by adjusting the pitchof the cholesteric liquid crystal.

The present invention also provides, in a second embodiment, areflective color liquid crystal display device, including first andsecond substrate spaced apart from and opposite to each other; a liquidcrystal layer interposed between the first and second substrates andhaving liquid crystal molecules and a phase difference λ/4; a linearpolarizer arranged over the first substrate, the polarizer converting anatural light a linearly polarized light; a retardation film arrangedunder the linear polarizer, the retardation film converting the linearlypolarized light a circularly polarized light; a negative uniaxial filmarranged between the retardation film and the liquid crystal layer, theuniaxial film compensating a phase difference between a direction of anincident light entering the liquid crystal layer and a direction of theliquid crystal molecules adjacent to the first and second substrates ofthe liquid crystal layer; a cholesteric liquid crystal color filterarranged under the liquid crystal layer, the color filter selectivelyreflecting the light from the liquid crystal layer; and a blackbackground arranged under the color filter, the black backgroundabsorbing the light passing through the color filter.

The liquid crystal molecules have a homeotropic orientation when anelectrical field is not applied.

The present invention also provides, in a third embodiment, a reflectivecolor liquid crystal display device, including a first substrate; asecond substrates spaced apart from and opposite to the fist substrate,the second substrate including a plurality of pixel electrode and commonelectrode being spaced apart from each other and being arranged thereon;a linear polarizer changing a natural light a linearly polarized lightand being positioned at an outer surface of the first substrate; aliquid crystal layer interposed between the first and second substratesand having liquid crystal molecules being oriented by a parallelelectric field between the pixel electrode and the common electrode; acholesteric liquid crystal color filter disposed between the liquidcrystal layer and the second substrate, selectively reflecting the lightfrom the liquid crystal layer; and a black background absorbing thelight passing through the color filter.

The reflective color liquid crystal display device further includes anegative uniaxial film arranged on the liquid crystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention,and, together with the description, serve to explain the principles ofthe invention:

FIG. 1 is a cross-sectional view illustrating a conventional reflectiveliquid crystal display;

FIG. 2 is a schematic view illustrating the state of light while itpasses through each component of a conventional reflective LCD devicewhen an electric field is not applied to the liquid crystal layer;

FIG. 3 is a schematic view illustrating the state of light while itpasses through each component of a conventional reflective LCD devicewhen an electric field is applied to the liquid crystal layer;

FIG. 4 is a graph illustrating the reflectivity with respect to thewavelength of the light that has passed through the color filter of theconventional liquid crystal display;

FIG. 5 is a cross-sectional view of a liquid crystal display accordingto a preferred embodiment of the present invention;

FIG. 6 is a schematic view illustrating the state of light while itpasses through each component in the normally white (NW) mode when anelectric field is not applied to the liquid crystal layer;

FIG. 7 is a schematic view illustrating the state of light while itpasses through each component in the NW mode when an electric field isapplied to liquid crystal layer;

FIG. 8 is a schematic view illustrating the state of light while itpasses through each component in the normally black (NB) mode when anelectric field is not applied to liquid crystal layer;

FIG. 9 is a schematic view illustrating the state of light while itpasses through each component in the NB mode when an electric field isapplied to the liquid crystal layer;

FIG. 10 is a graph illustrating the light transmissivity with respect tothe wavelength of an incident light a liquid crystal display of thepresent invention; and

FIG. 11 is a graph illustrating the light reflectivity with respect tothe wavelength of incident light a liquid crystal display according toan embodiment of the present invention;

FIG. 12 is a cross-sectional view showing a reflective color LCD deviceaccording a second preferred embodiment of the present invention;

FIG. 13A is a schematic view illustrating the state of light while itpasses through each component of the reflective color LCD deviceaccording to the second preferred embodiment of the present inventionwhen the LCD device is in the OFF state;

FIG. 13B is a schematic view illustrating the state of light while itpasses through each component of the reflective color LCD deviceaccording to the second preferred embodiment of the present inventionwhen the LCD device is in the ON state;

FIG. 14A is a cross-section view showing the conventional IPS LCD devicewhen a parallel electric field is not applied to the pixel electrode andthe common electrode;

FIG. 14B is a cross-section view showing the conventional IPS LCD devicewhen a parallel electric field is applied to the pixel electrode and thecommon electrode;

FIG. 15 is a cross-section view showing the reflective IPS LCD devicethat includes the CLC color filter and the negative uniaxial filmaccording to the third embodiment of the present invention;

FIG. 16A is a schematic view showing the state of light while it passesthrough each component of the reflective IPS LCD device when an electricfield is not applied; and

FIG. 16B is a schematic view showing the state of light while it passesthrough each component of the reflective IPS LCD device when an electricfield is applied.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, example of which is illustrated in theaccompanying drawings.

FIG. 5 shows a reflective LCD according to a first preferred embodimentof the present invention.

As shown in FIG. 5, first and second substrates 30 and 32 made of atransparent insulating material are spaced apart from and opposite toeach other.

A first electrode 42 is arranged on the bottom surface of the firstsubstrate 30. A retardation film 44 is disposed on the first substrate30, and a linear polarizer 46 is disposed on the retardation film 44.The retardation film 44 is preferably a λ/4 plate. A black background 34is formed on the bottom surface of the second substrate 32. The blackbackground 34 is preferably made of a polymer material that can absorblight. A cholesteric liquid crystal (CLC) layer is deposited on thesecond substrate 32 and patterned for a CLC color filter layer 36. Atransparent conductive metal layer is deposited on the CLC color filterlayer 36 and patterned into the second electrode 38. Then, after thefirst and second substrates are spaced apart from each other with apredetermined gap and aligned with each other, the liquid crystal isinjected into the gap.

The phase of light is either changed or not changed as it passes throughthe liquid crystal layer 40 according to the application of a voltage tothe liquid crystal layer, which is done through the first and secondelectrodes.

The CLC color filter 36 is a color filter made of CLC and it selectivelyreflects or transmits incident light. For example, if the molecularstructure of the CLC is twisted in the right direction, the color filter36 reflects only right-handed circularly polarized light. The CLC colorfilter 36 includes a plurality of pixels. Each pixel has threesub-pixels so that the reflected light is colored to red (R), green (G)and blue (B) colors and, therefore each color has a dominant wavelengthband leading to a high color purity.

As well known, all objects have their intrinsic wavelength, and thecolor that an observer recognizes is the wavelength of the lightreflected from or transmitted through the object. The wavelength rangeof visible light is about 380 nm to 780 nm. The visible light region canbe broadly divided into red, green, and blue regions. The wavelength ofthe red visible light region is about 660 nm, that of green is about 530nm, and that of blue is about 470 nm.

The pitch of the liquid crystal is controllable and, therefore it ispossible that the CLC color filter can selectively reflect light havingthe intrinsic wavelength of the color corresponding to each pixelthereby clearly displaying the colors of red (R), green (G) and blue (B)with a high purity.

The operation of the above-described LCD device will be described inmore detail with reference to FIGS. 6 to 9, in which light states areshown assuming that observer traces the light.

In order to implement a precise color, a plurality of the CLC colorfilters can be arranged, and the retardation film can also be arranged,not only on the first substrate but also on the second substrate. Atthis time, the retardation film is preferably arranged between theliquid crystal layer and the CLC color filter.

Further, a plurality of black backgrounds can be arranged on eithersurface of the second substrate to absorb unnecessary light.

FIG. 6 shows the state of light while it passes through each componentin the normally white (NW) mode device when an electric field is notapplied to liquid crystal layer.

The natural light is first converted into linearly polarized lightthrough the linear polarizer 46. The linearly polarized light is changedinto right-handed circularly polarized light through the retardationfilm 44. The right-handed circularly polarized light is converted againinto left-handed circularly polarized light through the liquid crystallayer 40, then reflected from the color filter 36 that is designed toreflect only left-handed circularly polarized light. The reflectedleft-handed circularly polarized light is converted into right-handedcircularly polarized light through the liquid crystal layer 36. Theright-handed circularly polarized light is finally converted intolinearly polarized light through the retardation film 44.

FIG. 7 shows the state of light while it passes through each componentin the NW mode device when an electric field is applied to liquidcrystal layer.

The natural light is first converted into linearly polarized lightthrough the linear polarizer 46. The linearly polarized light is changedinto right-handed circularly polarized light through the retardationfilm 44. The right-handed circularly polarized light passes through theliquid crystal layer 40 and the color filter 36 “as is”, and then isabsorbed by the black background 34 formed on either surface of thesecond substrate 32.

FIG. 8 shows the state of light while it passes through each componentin the NB mode device when an electric field is not applied to liquidcrystal.

The natural light is first converted into linearly polarized lightthrough the linear polarizer 46. The linearly polarized light is changedinto left-handed circularly polarized light through the retardation film44. The left-handed circularly polarized light is converted intoright-handed polarized light through the liquid crystal layer 40, andthen the right-handed polarized light passes through the color filter 36“as is” and is absorbed by the black background 34 formed on eithersurface of the second substrate 32.

FIG. 9 shows the state of light while it passes through each componentsin the NB mode device when an electric field is applied to the liquidcrystal layer.

The natural light is first converted into linearly polarized lightthrough the linear polarizer 46. The linearly polarized light is changedinto left-handed circularly polarized light through the retardation film44. The left-handed circularly polarized light passes through the liquidcrystal layer 40 “as is”, and then is reflected from the color filter 36which is designed to reflect only left-handed circularly polarizedlight. The reflected left-handed circularly polarized light passesthrough the liquid crystal layer 40 “as is”, and is finally convertedinto linearly polarized light through the retardation film 44.

FIG. 10 is a graph illustrating the relationship between the selectivetransmittance and the wavelength of light for the color filter, and FIG.11 is a graph illustrating the relationship between the selectivereflectance and the wavelength of light for the color filter. The graphof FIG. 10 can also be applied to a liquid crystal device containingyellow, magenta, and cyan dyes, regardless of whether the LCD device isthe transmissive or reflective type. Accordingly, a reflective LCDdevice which does not have a sufficient amount of light can be designedso as to display yellow, cyan and magenta, because these colors arerelatively higher in brightness than the three basic colors.

The CLC color filter transmits circularly polarized light having thesame orientation as that of the CLC, and reflects circularly polarizedlight having a different orientation from that of the CLC. That is,selective reflection of right- or left-handed circularly polarized lightdepends on the twisted direction of liquid crystal. For example, the CLCcolor filter can selectively reflect either right- or left-handedcircularly polarized light, depending upon the structural characteristicof the CLC molecules. The reflective LCD device according to preferredembodiments of the present invention provides colors using thecharacteristic of the CLC color filter described above.

As shown in FIG. 10, the X-axis indicates a wavelength of light, and theY-axis denotes transmissivity of light. The bandwidths C and D can becontrolled by adjusting the pitch of the CLC color filter. As describedabove, transmitted light is absorbed by the black background and cannotbe seen by the observer.

As shown in FIG. 11, the X-axis indicates the wavelength of the light,and the Y-axis denotes the reflective index of light. Likewise, thebandwidths E and F can be controlled by adjusting the pitch of the CLCcolor filter. As described above, the reflected circularly polarizedlight is displayed.

By properly adjusting the CLC color filter, it can transmit or reflectall colors except for a certain desired color, or alternatively, it canreflect or transmit only one color.

As described herein, the LCD device according to a first preferredembodiment of the present invention has the following advantages.

First, the CLC color filter selectively transmits light, and extraneouslight is absorbed effectively by the black background, leading to ahigher brightness. liquid crystal molecules of the homogeneousorientation change the direction of the polarized light. For example,right-handed circularly polarized light is changed to left-handedcircularly polarized light. As shown in FIG. 13B, light passes throughthe linear polarizer 123 and is changed to linearly polarized light. Thelinearly polarized light passes through the retardation film 121 and ischanged to right-handed circularly polarized light. The right-handedcircularly polarized light passes through the negative uniaxial film 119“as is” and passes through the liquid crystal layer 115 and is changedto left-handed circularly polarized light. The left-handed circularlypolarized light reflects from the CLC color filter 117 that is set toreflect left-handed circularly polarized light. The reflectedleft-handed circularly polarized light passes through the liquid crystallayer 115 again and is changed to right-handed circularly polarizedlight. The right-handed circularly polarized light passes through theretardation film 121 and is changed to linearly polarized light,parallel to the transmission axis (i.e., 45°), resulting in a whitedisplay screen.

At this time, when an electric field is applied to the liquid crystallayer 115, the liquid crystal molecules adjacent to the substratescontact the upper and lower substrates 11 and 113 due to the anchoringenergy (i.e., the orientation restriction force in the direction ofazimuth on the liquid crystal cell substrate plane), which is one of theimportant parameters of liquid crystal cells, and the long axis of theliquid crystal molecules becomes parallel to the direction of theelectric field. The negative uniaxial film 119 compensates the phasedifference between the light incident to the liquid crystal layer 115and the liquid crystal molecules on the surfaces of the substrates, sothe right-handed circularly polarized light from the retardation film121 can be induced to desirably pass through the liquid crystal layer115 and reflect on the CLC color filter 117.

The negative uniaxial film 119 can also be used in the mode that theliquid crystal molecules have the homogeneous orientation when theelectric field is not applied.

Second, since the CLC color filter is arranged in the lower part of theLCD device, multi-reflections of light in the upper part of the LCDdevice can be minimized.

Third, the color purity can be greatly improved because the color puritydoes not have to be sacrificed.

A second preferred embodiment of the present invention is directed toimprove the viewing angle and the color purity of the LCD device havingthe liquid crystal layer of a homeotropic orientation and the CLC colorfilter.

FIG. 12 shows the reflective color LCD device according the secondpreferred embodiment of the present invention. As shown in FIG. 12, thereflective color LCD device includes upper and lower substrates 111 and113 with the liquid crystal layer 115 interposed therebetween. The uppersubstrate 111 includes the polarizer 123, the retardation film 121 and anegative uniaxial film 119, which are stacked in the above-describedorder. The lower substrate 113 also includes the CLC color filter 117and the black background 124. Though not shown, a transparent pixelelectrode is arranged on the CLC color filter 117, and a commonelectrode is arranged on the bottom surface of the upper substrate 111such that the two electrodes apply an electric field to the liquidcrystal layer 115.

The controllable CLC color filter 117, according to the second preferredembodiment of the present invention, is preferably set to selectivelyreflect only right-handed polarized light. The negative uniaxial film119 serves to compensate the color purity and to improve the contrastratio and the viewing angle.

FIG. 13A shows the state of light while it passes through each componentof the reflective color LCD device according to the second preferredembodiment of the present invention when the LCD device is in the OFFstate. As shown in FIG. 13A, the incident light passes through thelinear polarizer 123 and is changed to linearly polarized light parallelto the transmission axis (i.e., 45°) of the linear polarizer 123. Thelinearly polarized light passes through the retardation film 121 and ischanged to right-handed circularly polarized light. The right-handedcircularly polarized light passes through the negative uniaxial film 119and the liquid crystal layer of the homeotropic orientation whosemolecules are vertical between the two substrates 111 and 113 “as is”.Then, the right-handed circularly polarized light passes through the CLCcolor filter 117 and then is absorbed by the black background 124,resulting in a black screen.

At this point, the uniaxial film 119 compensates an optical phasedifference according to a change in the anisotropy refractive index ofliquid crystals. That is, it serves to induce the right-handedcircularly polarized light from the retardation film 121 so that theright-handed circularly polarized light may pass through the CLC colorfilter 117 and then be effectively absorbed by the black background 124,leading to an improved dark characteristic of the reflective LCD device.

If the negative uniaxial film 119 described above is removed, the darkcharacteristic of the reflective LCD device is degraded, leading to anarrower viewing angle. This is because part of the right-handedcircularly polarized light incident to the liquid crystal molecules ofthe homeotropic orientation has an angle with respect to the long axisof the liquid crystal molecules and, therefore the phase of theright-handed circularly polarized light differs due to the phaseretardation, thereby changing the polarized light state. As a result,part of the right-handed circularly polarized light cannot betransmitted through the CLC color filter and reflects therefrom.Therefore, the negative uniaxial film 119 is used in order to compensatefor such a phase retardation.

FIG. 13B shows the state of light while it passes through each componentof the reflective color LCD device according to the second preferredembodiment of the present invention when the LCD device is in the ONstate. When an electric field is applied, the liquid crystal moleculeshave the homogeneous orientation parallel to the substrates, so the

A third preferred embodiment has a reflective in-plane switching (IPS)LCD device that includes the CLC color filter and the negative uniaxialfilm. The IPS LCD device has been introduced to obtain a wide viewingangle and has a structure in which the pixel electrode and the commonelectrode are arranged on the same plane.

FIG. 14A shows the conventional IPS LCD device when a parallel electricfield is not applied to both the pixel electrode 211 and the commonelectrode 213.

As shown in FIG. 14A, the IPS LCD device includes the upper and lowersubstrates 217 and 219 with the liquid crystal layer 215 a interposedtherebetween. Further, the upper polarizer 221 is arranged on the uppersubstrate 217, and the lower polarizer 223 is arranged on the bottomsurface of the lower substrate 223. The pixel electrode 211 and thecommon electrode 213 are both arranged on the lower substrate 219 andspaced apart from each other. The upper and lower substrates 217 and 219are made of a transparent conductive metal such as ITO (Indium TinOxide). Liquid crystal molecules are aligned horizontally in onedirection when voltage is not applied, blocking polarized light andresulting in a black screen. Because molecules are completelyhorizontal, viewing angle makes little difference.

FIG. 14B shows the conventional IPS LCD device when the parallelelectric field is applied to the pixel electrode 211 and the commonelectrode 213.

As shown in FIG. 14B, as voltage is applied to the electrodes, liquidcrystal molecules are horizontally rotated up to 90° to line up with thepolarizer. The light travels through the upper and lower polarizersuntwisted, resulting in a white screen.

FIG. 15 shows the reflective IPS LCD device that includes the CLC colorfilter and the negative uniaxial film according to the third embodimentof the present invention.

As shown in FIG. 15, the reflective IPS LCD device includes a polarizer321, a uniaxial film 318, upper and lower substrates 311 and 313 withthe liquid crystal layer 315 interposed therebetween, a CLC color filter319 on the lower substrate 313, and a black background 323. Though notshown, a plurality of pixel electrodes and common electrodes spacedapart from each other are arranged on the CLC color filter 319. Theliquid crystal is a nematic liquid crystal, and the liquid crystal layer315 acts as a quarter wave plate.

FIG. 16A shows the state of light while it passes through each componentof the reflective IPS LCD device when an electric field is not applied.

The liquid crystal molecules are horizontally aligned in thelongitudinal direction of the pixel electrode and the common electrodewhen an electric field is not applied. Therefore, the linearly polarizedlight from the polarizer 321 is changed to left-handed circularlypolarized light with a phase difference λ/4 after passing through thenegative uniaxial film 318 and the liquid crystal layer 315. Theleft-handed circularly polarized light reflects from the CLC colorfilter 319 and is directed to the liquid crystal layer 315. Then theleft-handed circularly polarized light from the liquid crystal layer 315is changed to linearly polarized light parallel to the transmission axisof the polarizer 321, resulting in a white screen.

FIG. 16B shows the state of light while it passes through each componentof the reflective IPS LCD device when an electric field is applied.

The liquid crystal molecules are horizontally rotated up to 90° to lineup with the polarizer 321 when the electric field is applied. Therefore,the liquid crystal layer 315 serves to change the linearly polarizedlight into right-handed circularly polarized light. The linearlypolarized light from the polarizer 321 passes through the liquid crystallayer 315 and is changed into right-handed circularly polarized light.The right-handed circularly polarized light is transmitted through theCLC color filter 319 “as is” and is absorbed by the black background323, resulting in a black screen. The negative uniaxial film 318 acts asa film for compensating the viewing angle as in the second preferredembodiment of the present invention.

As described herein, the reflective LCD device according to the secondand third preferred embodiments of the present invention have thefollowing advantages.

First, the color purity is greatly improved and the brightness of thelight becomes maximized due to the CLC color filter.

Second, a wide viewing angle and a high contrast ratio are attained dueto the negative uniaxial film.

Other embodiments of the invention will be apparent to the skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

For example, the color filter, black background, and retardation filmcan be positioned in other positions, or there can be more than one ofany of these elements. That is, the retardation film may be formed onthe cholesteric liquid crystal color filter on the second substrate, andmore than two color filters and black backgrounds may be provided.Further, the black background may be formed on the second substrate.

1. A reflective liquid crystal display comprising: a linear polarizerfor converting natural light into linearly polarized light; aretardation film for converting the linearly polarized light intocircularly polarized light; a liquid crystal layer for receiving thecircularly polarized light and varying the phase of the circularlypolarized light depending on the presence of an applied electric field;a cholesteric liquid crystal color filter for receiving the circularlypolarized light from the liquid crystal layer, and selectivelyreflecting the circularly polarized light received from the liquidcrystal layer; and a black background for absorbing a portion of lightpassing through the color filter.
 2. The reflective liquid crystaldisplay of claim 1, wherein the retardation film is a λ/4 plate.
 3. Thereflective liquid crystal display of claim 1, wherein the blackbackground is located beneath the color filter.
 4. The reflective liquidcrystal display of claim 1, wherein the retardation film is locatedbetween the linear polarizer and the color filter.
 5. The reflectiveliquid crystal display of claim 1, wherein the black background is madeof a polymeric material.
 6. The reflective liquid crystal display ofclaim 1, wherein a bandwidth of the color filter can be controlled byadjusting a pitch of the cholesteric liquid crystal color filter.
 7. Areflective liquid crystal display comprising: first and secondsubstrates opposite to and spaced apart from each other; a liquidcrystal layer interposed between the first and the second substrates,the liquid crystal layer having a first switching mode in which a phaseof light is changed while passing through it, and a second switchingmode in which the phase of light is not changed while passing throughit; first and second electrodes for applying an electric field to theliquid crystal layer; a semiconductor element located on the secondsubstrate for switching an electric signal applied to the liquid crystallayer; a retardation film located on the first substrate for convertinglinearly polarized light into circularly polarized light; a linearpolarizer located on the retardation film, for converting natural lightinto the linearly polarized light; a cholesteric liquid crystal colorfilter located on the second substrate for selectively reflecting lighthaving at least one color received from the liquid crystal layer; and ablack background located beneath the second substrate for absorbinglight passing through the cholesteric liquid crystal color filter. 8.The reflective liquid crystal display of claim 7, wherein theretardation film is a λ/4 plate.
 9. The reflective liquid crystaldisplay of claim 7, wherein the black background is made of a polymericmaterial.
 10. The reflective liquid crystal display of claim 7, whereina bandwidth of the color filter can be controlled by adjusting a pitchof the cholesteric liquid crystal color filter.